While CRTs have been the mainstream of conventional general use display devices, recent years have seen the increasing use of active-matrix type liquid crystal display devices (hereinafter referred to as the “LCDs”). An LCD is a display device utilizing the light transmittance of liquid crystals. An LCD itself does not emit light; gray scale display is accomplished by controlling the light of a backlight on the back surface of the LCD between transmission, shut-off, and an intermediate state therebetween.
While LCDs have been mainly applied to the screen of notebook PCs and desktop PC monitors, in recent years, LCDs are beginning to be used as TVs. Since use of LCDs as TVs is subject to strict requirements in terms of brightness or that the color does not change no matter from which direction the display is viewed (wide viewing angle), the applicable liquid crystal display modes are limited.
The transmission characteristics and viewing angles of the liquid crystal display modes that have been published to date are concisely summarized in IDRC′ 03 P.65 Uchida. et al.
Further, as a video display device for use as a TV, not only faithful reproduction of a display object but also achieving beautiful display is important. For instance, a TV using the CRT achieves a dynamic range higher than the full-screen white contrast ratio by utilizing white peak display characteristics.
The white display luminance of an LCD is determined by the luminance of the backlight and the transmittance of liquid crystals. Since enhanced luminance of the backlight leads to increased power consumption, it is desirable to improve the transmittance of liquid crystals.
As a method of realizing white peak display by substantially improving the transmittance of liquid crystals and thus increasing white luminance, as described in Japanese Patent Laid-Open No. 2000-147666 or Japanese Patent Laid-Open No. 2001-154636, for example, there is one aimed at improving the transmittance characteristics without increasing power consumption, by using a white-color (hereinafter, referred to as “W”) pixel in addition to pixels of the three primary colors of red, green, and blue (hereinafter referred to as “R, G, and B”).
Further, Japanese Patent Laid-Open No. 2002-149116 describes about switching between RGB display and RGBW display for a part of the area within the screen or on a screen-by-screen basis.
It should be noted that in a liquid crystal display device of the RGBW pixel structure as well, the image data signal to be input consists of only RGB, so it is necessary to carry out conversion from RGB image data to RGBW image data.
Here, making image display including white color inevitably results in image degradation due to chromatic purity degradation. In view of this, there have been proposed numerous RGB-RGBW conversion methods for making such image degradation relatively small and less conspicuous (see Japanese Patent Laid-Open Nos. 2001-147666, 2001-154636, 2002-149116, 2003-295812, and 2004-102292).
On the other hand, as a method of expanding the dynamic range of a display image, for example, there is one in which, as described in U.S. Pat. No. 3,215,400 below, the contrast and the luminance of the backlight are adjusted in a dynamic fashion in accordance with the input image data to be displayed, or one in which, as described in Japanese Patent Laid-Open No. 2002-41004 or Japanese Patent Laid-Open No. 2002-333858, the gray scale-luminance characteristics (hereinafter referred to as the “γ characteristics”) are controlled through analysis of the input image data to be displayed, thereby achieving video display with sharp contrast.
It should be noted that the term white peak mentioned above refers to a display part of a level higher than that of normal white display due to light reflection or the like such as caused by metallic luster or water droplets within the display image. For such white peak display, dedicated data areas are specified by the NTSC or Hi-Vision standards that are television broadcast standards.
For example, in ITU-R Recommendation 709-5 which is an international Hi-Vision standard, when representing a signal of R, G, B, or Y (luminance level) by 10 bits of 0 to 1023, the image data range is set as 4 to 1019 (the rest being used as a timing signal), of which the black level is specified as 64 and normal white (nominal peak) is specified as 940. That is, the range from 940 to 1019 of the data area is a data area for white peak higher than normal white=100% white (it should be noted that the range from 64 to 4 is at the same black level throughout).
However, in the case of a TV using a liquid crystal display device or a so-called liquid crystal TV, as described above, using the RGBW structure as described in Japanese Patent Laid-Open Nos. 2001-147666, 154636/2001, 149116/2002, 295812/2003, and 102292/2004 in order to improve white display luminance without increasing power consumption inevitably results in image degradation due to chromatic purity degradation.
For example, while Japanese Patent Laid-Open No. 2001-147666 describes means for displaying an image while achieving improved luminance and without causing changes in the chromaticity at gray levels, the document also describes that such conversion is not possible for all the gray level regions but is possible only for the region as shown in FIG. 5 of Japanese Patent Laid-Open No. 2001-147666 mentioned above.
In regions other than this region, it is necessary to sacrifice either the chromaticity or the luminance enhancement factor; when display data outside this region is included in a normal image, the chromaticity or luminance enhancement factor of the corresponding pixel differs from that in other portions, resulting in an image failure.
It should be noted that color degradation can be made inconspicuous to some degree by using the conversion method described in each of Japanese Patent Laid-Open Nos. 2001-154636, 2002-149116, 2003-295812, and 2004-102292. However, when displaying the brightest primary color, the above-mentioned conversion method is not effective.
For instance, when white color is mixed into the brightest red color or the like for display, this always results in color degradation. The degree of degradation is readily discernable to an extent such that color degradation can be visually discerned even with the slightest admixture of white color.
As described above, although display in RGBW allows an improvement in luminance without an increase in power consumption, this is inevitably accompanied by color degradation, especially in the case of a bright image; the conversion method or the mastering of the technique is thus difficult, and hence there have not been many applications of the technique to the actual products.